The latest bouillabaisse to bite the dust is at Le Provenc,al, one of Washington's oldest and grandest French restaurants. After 17 years as a local fixture, the restaurant closed its doors and has been sold to Lenny Lowenstein, who plans to strip Le Provenc,al of its French character and turn it into Board Room II, complete with dancing girls who may be topless, slated to open sometime in January.

It won't be Lowenstein's first restaurant turnaround. He transformed his original Board Room on Vermont Avenue, which he described as a "class executive fun spot" with a lunch-time comedian, buffet and scantily clad waitresses who danced on the tables, into Lenny's Restaurant, a business-lunch restaurant with no dancing girls.

Now he plans to revive the original Board Room idea on 20th Street, calling it a "glorified Playboy concept." Lowenstein outlined plans to house conference and seminar rooms, a billiard room and a sports salon on the premises along with an eating section. That will consist of an American continental supper club with dancing and entertainment in the evenings and the old Board Room scheme of a buffet with comedy show at lunch.

For waitress and showgirl costumes, Lowenstein has gone the Las Vegas plume-and-sequins route. He has hired a designer from Atlantic City, he said. Tentatively, his idea is to have the girls dance daily, perhaps topless. Lowenstein said he wants Board Room II "to be risque', but doesn't want [it] to be classified as a girlie place." At first, the new restaurant and club, he said, will be open to the public, but he hopes eventually to make it private.

Le Provenc,al's death came after a long illness. Jacques Blanc, one of Washington's most celebrated chefs and owner of Le Provenc,al from its founding, sold the restaurant three years ago to Matthew Korson. Blanc left it the following year, then opened his own cooking school. Since Blanc's departure, Korson has tried innovations including opening for breakfast.

Korson cited Reaganomics and the costs of the restaurant union as reasons for closing. "Unions are good things to have for big organizations, but not for small ones. Then, you can't control them," said Korson.

Although both Korson and Blanc said they're sad to see the restaurant go under, they agreed that there are too many downtown French restaurants in Washington. "Perhaps it's a good thing that you reduce the number," said Blanc. "It would be better for the others."

The demise of Le Provenc,al follows that of two other prominent downtown French restaurants, Sans Souci, which has been reborn Italian under the same name, and Le Pavillon, which is turning Chinese and about to reopen as the Sichuan Pavilion.

Matthew Korson said he was not aware of Lowenstein's plans for the restaurant, but was just told he was opening a private club. The payment for the restaurant is in cash, said Korson, "so I don't care. I just wish him the best of luck."